This invention relates to dielectric block filters for radio-frequency signals, and in particular, to monoblock single pass-band and duplexing filters.
Ceramic block filters offer several advantages over lumped component filters. The blocks are relatively easy to manufacture, rugged, and relatively compact. In the basic ceramic block filter design, the resonators are formed by typically cylindrical passages, called holes, extending through the block from the long narrow side to the opposite long narrow side. The block is substantially plated with a conductive material (i.e. metallized) on all but one of its six (outer) sides and on the inside walls formed by the resonator holes.
One of the two opposing sides containing through-hole openings is not fully metallized, but instead bears a metallization pattern designed to couple input and output signals through the series of resonators. This patterned side is conventionally labeled the top of the block. In some designs, the pattern may extend to sides of the block, where input/output electrodes are formed.
The reactive coupling between adjacent resonators is dictated, at least to some extent, by the physical dimensions of each resonator, by the orientation of each resonator with respect to the other resonators, and by aspects of the top surface metallization pattern. Interactions of the electromagnetic fields within and around the block are complex and difficult to predict.
These filters may also be equipped with an external metallic shield attached to and positioned across the open-circuited end of the block in order to cancel parasitic coupling between non-adjacent resonators and to achieve acceptable stopbands.
Although such RF signal filters have received wide-spread commercial acceptance since the 1980s, efforts at improvement on this basic design continued.
In the interest of allowing wireless communication providers to provide additional service, governments worldwide have allocated new higher RF frequencies for commercial use. To better exploit these newly allocated frequencies, standard setting organizations have adopted bandwidth specifications with compressed transmit and receive bands as well as individual channels. These trends are pushing the limits of filter technology to provide sufficient frequency selectivity and band isolation.
Coupled with the higher frequencies and crowded channels are the consumer market trends towards ever smaller wireless communication devices (e.g. handsets) and longer battery life. Combined, these trends place difficult constraints on the design of wireless components such as filters. Filter designers may not simply add more space-taking resonators or allow greater insertion loss in order to provide improved signal rejection.
A specific challenge in RF filter design is providing sufficient attenuation (or suppression) of signals that are outside the target passband at frequencies which are integer multiples of the frequencies within the passband. The label applied to such integer-multiple frequencies of the passband is a xe2x80x9charmonic.xe2x80x9d Providing sufficient signal attenuation at the third (3rd) harmonic has been a persistent challenge.
Therefore, it would be desirable to provide an RF filter that better attenuates 3rd harmonic frequencies without sacrificing other performance parameters such as size, passband insertion loss and material costs.
This invention overcomes problems of the prior art by providing a ceramic block RF filter having improved 3rd harmonic rejection in a small size. A duplexing communication signal filter for connection to an antenna, a transmitter and a receiver for filtering an incoming signal from the antenna to the receiver are provided. The filter also modulates an outgoing signal from the transmitter to the antenna. The filter includes a core of dielectric material having a top and bottom surface and four side surfaces having vertical edges. Several through holes extend from the top to the bottom surface. Each of the through holes defines a resonator. A contiguous unmetallized area is located on the top surface and extends to one of the side surfaces. A first metallization area is located on the bottom surface, the side surfaces and on the inner surfaces of the through holes. A transmitter electrode, an antenna electrode and a receiver electrode are located on the top surface and extend to one of the side surfaces. Several metallized resonator pads are adjacent to each of the through holes on the top surface. The resonator pads are connected to the first metallization area. A metallized serpentine region extends from at least one of the resonator pads toward one of the side surfaces. The metallized serpentine region is adapted to cause attenuation of a third harmonic frequency.
An embodiment of the present invention can be specified as a duplexing communication signal filter adapted for connection to an antenna, a transmitter and a receiver for filtering an incoming signal from the antenna to the receiver and for filtering an outgoing signal from the transmitter to the antenna. The duplexing filter comprises a rigid core of dielectric material with top, bottom and at least four side surfaces and a surface-layer pattern of metallized and unmetallized areas on the core.
The rigid core of dielectric material defines a series of through-holes, each extending from an opening on the core top surface to an opening on the core bottom surface. The surface-layer pattern of metallized and unmetallized areas includes a wide area of metallization for providing off-band signal absorption, a pad of metallization adjacent to at least one of the through-hole openings on the top surface (i.e. a resonator pad), a contiguous unmetallized area substantially surrounding the resonator pad, a transmitter connection area of metallization, a receiver connection area of metallization spaced apart from the transmitter connection area, and an antenna connection area of metallization positioned between the transmitter connection area and the receiver connection area. The resonator pad has a narrow intricate extension. The intricate extension preferably has a sinuous path.
In an alternate embodiment of the present invention a signal filter having input and output electrodes is provided. Specifically the filter comprises a rigid core of dielectric material, preferably with a rectangular parallelepiped shape, and a surface-layer pattern of metallized and unmetallized areas supported by the core. The core has a top surface, bottom surface and at least four side surfaces. The core defines a series of through-holes, each extending from an opening on the top surface to an opening on the bottom surface. The surface-layer pattern of metallized and unmetallized areas includes a wide area of metallization to absorb off-band signals, a pad of metallization adjacent at least one of the through-hole openings on the top surface. The pad has a narrow sinuous extension. The surface-layer pattern also includes a contiguous unmetallized area substantially surrounding the pad, an input connection area of metallization and an output connection area of metallization spaced apart from the input connection area.